This invention relates to an optical disk apparatus for optically recording and receiving data to and from an optical disk.
When the optical disk apparatus records or reproduces data, it requires a reference signal to ensure synchronization of a recording signal and a reproducing signal. To obtain a reference signal, an index mark is put on a non-recording area of an optical disk. This mark is detected by a photosensor while the disk is rotating. Every time the disk rotates once, the photosensor generates one or more signals. Reflective material is vapor-deposited on the recording area of the optical disk. A spiral recording track is formed on the vapor-deposited layer, and data is optically recorded in this track. The data may be optically reproduced from the disk. When the reflectivity of the layer changes, the level of a signal reproduced from the disk changes, and so does the peak value of an error signal, e.g., a tracking error signal. The change in the peak value of the error signal results in the change in the level of an output of a servo system which contains focussing and tracking servo controls. If the level of the output of the system varies, the system fails to operate stably. An amplifier which has a gain large enough to stabilize the servo system is used. However, when the reflectivity of the reflective layer is too large to produce a high level signal is too high. An excessive gain creates various problems hindering the stable operation of the servo system. Another problem is inherent in this type of an optical disk apparatus. Binary data is recorded on the track in the form of a series of pits. A laser light reflected from this track is thus diffracted. Due to the diffraction, part of the reflected light fails to reach an objective lens. The amount of light which the objective lens receives is therefore reduced. This reduction of light varies in accordance with the depths of the pits. Consequently, the data representing the reflectivity of the layer cannot be accurately obtained from the optical signals which are read from the disk and which correspond to the binary data recorded on the recording area of the disk.